1. Field of the Invention
The present invention relates to an ultrasonic flaw detector and an ultrasonic flaw detecting method.
2. Description of the Related Art
Ultrasonic flaw detection testing is a non-destructive technology that enables checking of the soundness of the surface and the inside of a construction material, and is a testing technology indispensable in various fields. Especially, in recent years, there is a demand for testing of a construction having a complex surface shape such as a curved surface, and the technological demand regarding ultrasonic flaw detection is becoming higher.
In the case an object to be inspected has a complex surface shape, there is a problem that an ultrasonic wave cannot be appropriately emitted on the object. For example, at a weld line and its heat-affected zone, a portion which is designed to be planar unintentionally becomes non-planar due to a strain or chevron-like deformation caused by welding heat input or a convex shape formed after molten metal is poured.
Furthermore, various piping structures typified by a nozzle stub of, for example, a nuclear power plant or a thermal power plant, or a platform region of a turbine blade is designed to have a complex shape, and even if constructed as designed, testing thereof is difficult. If such object is taken as the object of the ultrasonic flaw detection testing, an ultrasonic wave cannot be emitted on the object or, even if the ultrasonic wave is emitted, a desired refraction angle may not be obtained.
In contrast, a phased array (PA) is a technology of forming a given waveform by arranging small ultrasonic sensors, and varying the timing (delay time) of the sensors and emitting ultrasonic waves. Compared to a monolithic probe that can only emit ultrasonic waves at a predetermined angle, the phased array can possibly cope with a complex shape.
However, with the phased array technology, the delay time reflecting the shape has to be calculated for each target. Further, the value of the shape to be reflected should include not only the values on a drawing, but also the as-built values (a drawing created based on existing conditions and analysis data). Accordingly, to carry out ultrasonic flaw detection on a complex-shaped region, a technology for measuring the surface shape of a target with high accuracy is needed, as well as a technology for controlling ultrasonic waves (calculating the delay time) according to the curvature.
To solve the above matters, there is conventionally proposed a method which measures the surface shape of an object by an ultrasonic probe, optimizes the transmission delay time of the PA according to the measured shape, and conducts testing (for example, Japanese Patent Laid-Open No. 2007-170877: Patent Document 1).
However, the Patent Document 1 only describes optimization of delay time conditions according to the surface shape of an object. Therefore, when using an electronic scan, which, typified by a linear scan, sequentially moves elements to be used, elements to be used are fixed with respect to a focal point, and thus, there arises a problem that an ultrasonic wave enters from an unintended incident point. There is also provided a problem of an ultrasonic wave not reaching a test portion due to use of an element at a blind angle.
On the other hand, emitting an ultrasonic wave according to the surface shape allows to cope with the problem of the incident angle of the ultrasonic wave changing. However, at the time of displaying a flaw detection result, a flaw indication echo is displayed at a portion different from the actual detection position in a flaw detection result that is displayed in a case if analysis has been performed without taking the influence of the surface shape into consideration.
The position of an indication echo cannot be accurately identified unless the flaw detection result is separately corrected in consideration of the influence of the surface shape, and an error occurs in the detection position. That is, the flaw detection accuracy is greatly reduced. It is also conceivable that an indication echo is extended and is displayed with a shape different from the actual flaw. This can be overcome if an inspector corrects the detection position error by hand calculation while taking the influence of the surface shape into consideration. But this will increase the burden on the inspector. In addition, if the surface shape is complex, correction will require much skill.